Next generation spread-spectrum communication systems incorporate both a direct-spread and a multi-carrier option for transmission. One such communication system currently being developed with direct-spread and multicarrier capabilities is the next generation Code-Division Multiple-Access (CDMA) cellular communication system, more commonly referred to as cdma2000, or IS95C.
During direct-spread transmission the communication systems transmit information over the bandwidth of interest by using a single short code. Unlike direct-spread transmission, multi-carrier transmission transmits information over the bandwidth of interest by using multiple short codes. For multi-carrier transmission, each portion of the user's data that is spread with a single short code is transmitted within its own frequency band. Such a multi-carrier transmitter and receiver is shown in FIG. 1. As shown, data stream 101 enters demultiplexer 103, where it is sent to three separate spreaders 105. Each spreading circuitry 105 utilizes its own, unique Walsh code as well as a common pseudo-noise (PN) code When spreading data stream 101. After low-pass filtering and digital-to-analog conversion, each data path is mixed to a separate frequency by mixers 107. The output of mixers 107 is a portion of data stream 101, mixed to a unique frequency. After bandpass filtering, the output from mixers 107 is summed by summer 109 and transmitted as signal 121 comprising three separate frequency bands, each frequency band utilizing a unique short code. During reception, the three separate frequency bands 121 are received by receiver 120 and downconverted to a single frequency by mixers 125. After filtering and analog-to-digital conversion, the three signal paths are despread utilizing three separate despreaders (not shown), each using a similar PN code and a unique Walsh code.
The use of multiple spreaders 105 in multi-mode transmitters, makes multi-mode devices very expensive. Therefore a need exists for a method and apparatus for spreading and despreading data in a communication system utilizing both direct-spread and multi-carrier transmission, that is less costly than prior-art multi-mode transmitters and receivers.